Copyright laws exist in virtually every country of the world to protect artists and authors from those who would seek to profit by separating a work from the identity of its creator. The proliferation of the Internet has greatly facilitated the piracy and distribution of audio-visual works. Reproducing a counterfeit work and selling it as an original has never been easier.
At the end of 1997, the International Federation of the Phonographic Industry (IFPI), the British Phonographic Industry, and the Recording Industry Association of America (RIAA) engaged in a project to survey the extent of unauthorized use of music on the Internet. The initial search indicated that at any one time there could be up to 80,000 infringing MP3 files on the Internet. The actual number of servers on the Internet hosting infringing files was estimated to be 2,000 with locations in over 30 countries around the world. Since that survey, the availability of and interest in the digital music on the Internet has increased many times over.
The problem is not limited to audio works. The increasing popularity of streaming video, digital images, and all types of multimedia content has led to increased piracy of these types of works as well. Each day, the wall impeding the reproduction and distribution of infringing content (e.g., audio files, video files, digital images, etc.) gets shorter and weaker.
As a result, pirates effectively rob artists and authors of their lawful compensation. Unless technology provides for those who create artistic works to be compensated, both the creative community and the culture at large will be impoverished.
Identifying a Copyrighted Work
Unlike cassette tapes, CDs or DVDs, a digital content file has no jewel case, label, sticker, or the like on which to place a copyright notice that identifies the author. A digital content file is merely a set of binary data without a detectible and unmodifiable label. Thus, artists and authors are unable to affix a copyright notice to a work so that the public is put on notice that the work is protected by copyright law. Furthermore, such artists and authors are unable to inform the public of any additional information, such as the identity of the copyright holder or terms of a limited license.
Digital Tags
The music industry and trade groups are especially concerned by digital recording because there is no generation loss in digital transfers—a copy sounds the same as the original. Without limits on unauthorized copying, a digital audio recording format could easily encourage the pirating of master-quality recordings.
One solution is to append an associated digital “tag” that identifies the copyright holder to each audio file. To implement such a plan, all devices capable of such digital reproduction must faithfully reproduce the associated tag.
With the passage of the Audio Home Recording Act of 1992, inclusion of serial copying technology became law in the United States. This legislation mandated the inclusion of serial copying technology, such as SCMS (Serial Copy Management System), in consumer digital recorders. SCMS recognizes a “copyright flag” encoded on a prerecorded original (such as a CD), and writes that flag into the subcode of digital copies (such as a transfer from a CD to a DAT tape). The presence of the flag prevents an SCMS-equipped recorder from digitally copying the copy, thus breaking the chain of perfect digital cloning.
However, subsequent developments—both technical and legal—have demonstrated the limited benefits of this legislation. While digital-secure-music-delivery systems (such as SCMS) are designed to support the rights of content owners in the digital domain, the problem of analog copying requires a different approach. In the digital domain, information about the copy status of a given piece of music may be carried in the subcode, which is separate information that travels along with the audio data. In the analog domain, there is no subcode—the only place to put the extra information is to hide it within the audio signal itself.
Digital Watermarks
Techniques for identifying copyright information of digital content that address both analog and digital copying instances have received a great deal of attention in both the industrial community and the academic environment. One of the most promising “digital labeling” techniques is the incorporation of a digital watermark into the audio/video signal itself by altering the signal's frequency spectrum such that the perceptual characteristics of the original recording are preserved. In other words, a watermark is clandestinely integrated with a content sequence so that when copied, the watermark will be reproduced along with the sequence itself.
In general, a “digital watermark” is a pattern of bits inserted into a digital representation (i.e., signal or file) of content (i.e., an image, audio, video, or the like) that identifies the content's copyright information (e.g., author, rights, etc.). The name comes from the faintly visible watermarks imprinted on stationery that identify the manufacturer of the stationery. The purpose of digital watermarks is to provide copyright protection for intellectual property that is in digital format.
Unlike printed watermarks, which are intended to be somewhat visible, digital watermarks are designed to be completely invisible, or in the case of audio clips, inaudible. That is invisible to all except a specifically designed watermark detector. Moreover, the actual bits representing the watermark are typically scattered throughout the file in such a way that they cannot be identified and manipulated. Finally, the digital watermark should be robust enough so that it can withstand normal changes to the file, such as reductions from compression algorithms.
Satisfying all these requirements is no easy feat, but there are several competing technologies. All of them work by making the watermark appear as noise—that is, random data that exists in most digital files anyway. To view a watermark, you need a special program or device (i.e., a “detector”) that knows how to extract the watermark data.
Herein, such a digital watermark may be simply called a “watermark.” Generically, it may be called an “information pattern of discrete values” or a “data pattern of discrete values.” The content sequence (or signal) in which a watermark is encoded is effectively “noise” in relation to the watermark.
Watermarking
Watermarking gives content owners a way to self-identify each content sequence, thus providing proof of ownership and a way to track public performances for purposes of royalty distribution. It may also convey instructions, which can be used by a recording or playback device, to determine whether and how the content may be distributed. Because that data can be read even after the content has been converted from digital to an analog signal, watermarking can be a powerful tool to defeat analog circumvention of copy protection.
The general concept of watermarking has been around for at least 30 years. It was used by companies (such as Muzak™) to audibly identify music delivered through their systems. Today, however, the emphasis in watermarking is on inaudible approaches. By varying signals embedded in analog audio programs, it is possible to create patterns that may be recognized by consumer electronics devices or audio circuitry in computers.
Goals of Watermarking Technology
Watermarking technology has several highly desirable goals to facilitate protection of copyrights of content publishers. Below are listed several of such goals.
Perceptual Invisibility. The embedded information should not induce humanly perceptible changes in the quality of the resulting watermarked signal.
Statistical Invisibility. The embedded information should be quantitatively imperceptive for any exhaustive, heuristic, or probabilistic attempt to detect or remove the watermark. The complexity of successfully launching such attacks should be well beyond the computational power of publicly available computer systems.
Tamper-proofness. An attempt to remove the watermark should damage the value of the content well above the hearing or visual threshold.
Cost Effectiveness. The system should be inexpensive to license and implement on both programmable and application-specific platforms.
Non-disclosure of the Original. The watermarking and detection protocols should be such that the process of proving content copyright both in-situ and in-court, does not involve usage of the original recording.
Enforceability and Flexibility. The watermarking technique should provide strong and undeniable copyright proof. Similarly, it should enable a spectrum of protection levels, which correspond to variable audio/visual presentation and compression standards.
Resilience to Common Attacks. Public availability of powerful digital sound editing tools imposes that the watermarking and detection process is resilient to attacks spawned from such consoles. The standard set of plausible attacks is itemized in the Request for Proposals (RFP) of IFPI (International Federation of the Phonographic Industry) and RIAA (Recording Industry Association of America). The RFP encapsulates the following security requirements:                two successive D/A and A/D conversions,        data reduction coding techniques such as MP3,        adaptive transform coding (ATRAC),        adaptive sub-band coding,        Digital Audio Broadcasting (DAB),        Dolby AC2 and AC3 systems,        applying additive or multiplicative noise,        applying a second Embedded Signal, using the same system, to a single program fragment,        frequency response distortion corresponding to normal analogue frequency response controls such as bass, mid and treble controls, with maximum variation of 15 dB with respect to the original signal, and        applying frequency notches with possible frequency hopping.Watermark Circumvention        
If the encoding of a watermark can thwart a malicious attack, then it can avoid the harm of the introduction of unintentional noise. Therefore, any advancement in watermark technology that makes it more difficult for a malevolent attacker to assail the watermark also makes it more difficult for a watermark to be altered unintentionally.
In general, there are two common classes of malevolent attacks:                1. De-synchronization of watermark in digital signals. These attacks alter signals in such a way to make it difficult for the detector to identify the location of the encoded watermark codes.        2. Removing or altering the watermark. The attacker discovers the location of the watermark and intentionally alters the content sequence to remove or deteriorate a part of the watermark or its entirety.Framework to Thwart Attacks        
Accordingly, there is a need for a framework of protocols for hiding watermarks in digital signals that are effective against malevolent attacks. The framework should also be flexible to enable a spectrum of protection levels, which correspond to variable audio/visual presentation and compression standards, and yet resilient to common attacks spawned by powerful digital sound editing tools.
However, such a framework should support quick, efficient, and accurate detection of watermarks by a specifically designed watermark detector. Moreover, it is desirable for such a framework to minimize false indications of a watermark's presence or absence. Furthermore, it is best if the act of detection does not provide decipherable clues to a digital pirate as to the value or location of the embedded watermark.
Watermark Detection
The watermark detection process is performed by synchronously correlating the suspected content sequence with the watermark of the content publisher. A common pitfall for all watermarking systems that facilitate this type of data hiding is intolerance to desynchronization attacks (e.g., sample cropping, insertion, repetition, variable pitch-scale and time-scale modifications, audio restoration, and arbitrary combinations of these attacks) and deficiency of adequate techniques to address this problem during the detection process.
Furthermore, it is desirable to have a highly accurate, quick, and efficient watermark detection system. When detecting a watermark, the content of the sequence (e.g., video) is merely noise in relation to the watermark. Therefore, this “noise” hinders with such accurate, quick, and efficient watermark detection. However, of course, the watermark's purpose is to protect this “noise.”
Moreover, the mere act of accurately detecting a watermark in a signal may aid a digital pirate in empirically ascertaining the watermark. Conventionally, this risk is considered small and too difficult to address; therefore, the industry lives with this risk.
Traditional (Symmetric) Watermarking
FIG. 1 is a diagram showing a traditional watermarking technique utilized in a transaction between a copyright owner 100 and a client 102 over the Internet 104. A watermark generator 106 provides a digital watermark 108 that is incorporated into a content sequence 110 in a watermarking module 111 to form a watermarked content sequence 112. The watermarked content sequence 112 is uploaded to the Internet 104 and, ultimately, is downloaded to a computer 114 of the client 102. The computer 114 stores an application 116 that is used to determine if a content sequence contains the watermark 108. To do this, a watermark detector 118 in the application 116 has access to a copy of the watermark 108′ stored on the computer 114. The watermark detector 118 applies a correlation test with the watermarked content sequence 112 and the watermark 118′ to determine if the watermarked content sequence 112 contains the watermark 118′.
The watermark 108′ must be stored on the computer 114 in such a way that it remains secret, for if a malevolent attacker has a copy of a watermark, the attacker can simply subtract the watermark from a watermarked content sequence and have an unmarked content sequence. However, it is impossible to store the watermark 108′ on the computer 114 in such a way that it cannot be discovered. Furthermore, there is a problem of keeping the watermark 108 secret when transmitting the watermark 108 from the copyright owner 100 to the computer 114 at the client 102.
For this reason, traditional watermarking schemes have failed to provide a reliable way to incorporate a watermark within a content sequence and detect the watermark in the content sequence without providing an attacker with the key to extracting the watermark.